Thursday, February 24, 2022

ECG Blog #286 — Challenge: What is the Rhythm?

The long lead II rhythm strip shown in Figure-1 — was obtained from a 60-year old man, who presented with an acute pulmonary problem. He was hemodynamically stable at the time this tracing was obtained. No known prior cardiac history.
  • Challenge: How would you interpret this tracing?
  • Is there complete AV Block?

Figure-1: Long lead II rhythm strip that was sent to me.


NOTE: Some readers may prefer at this point to listen to the 7:40 minute ECG Audio PEARL before reading My Thoughts regarding the ECG in Figure-1. Feel free at any time to review to My Thoughts on this tracing (that appear below ECG MP-51a).


Today’s ECG Media PEARL #51a (7:40 minutes Audio) — Reviews of "Some Simple Steps to Help Interpret Complex Rhythms" ).


My APPROACH to the Rhythm in Figure-1:
As always, once I’ve ensured that my patient is hemodynamically stable — I favor use of the Ps, Qs, 3Rs Approach for systematic Rhythm Interpretation (See ECG Blog #185 for review of this system):
  • As I often emphasize — it does not matter in what sequence you address the 5 Parameters in the Ps, Qs, 3R Approach — as long as you always look for them all. As a result — I often change the sequence, depending on which of the parameters are easiest to assess.

My initial impression of the rhythm in Figure-1 (which I arrived at within seconds of looking at this tracing) — was the following.
  • Although we are only given a single lead to look at — the QRS complex clearly looks narrow (ie, not more than 0.10 second in duration). Therefore, the rhythm is supraventricular.
  • The ventricular Rate is not overly fast (ie, about 70-80/minute).
  • The rhythm looks almost, but not completely Regular (an observation that I wanted to confirm as soon as I had a moment to use calipers).

  • Lots of P waves are present! Some of these P waves are easy to see — as they appear before a number of QRS complexes, with PR intervals that could clearly be conducting. But other P waves are either hidden within the ST-T wave (ie, notching the T wave of beat #2) — or — appear before QRS complexes with a PR interval that is clearly too short to conduct (ie, as occurs for the P waves before beats #3, 7, 9, 11 and 13). And, no P wave at all appears before beat #5.

  • Regarding the 5th Parameter (ie, whether at least some P waves are Related to neighboring QRS complexes) — this was admittedly difficult to determine in my initial (ie, less than 10-second) assessment of the rhythm.

PEARL #1: The most helpful clues for me to determine whether any P waves are conducting (and if so, which ones?) — are the following: i) Look to see if any R-R intervals are clearly much shorter than the other R-R intervals (since the most common reason for there to be much shorter-than-expected R-R intervals — is that the P waves that precede such shorter intervals are being conducted); and, ii) Look to see if there are any PR intervals that repeat — since the finding of PR intervals that repeat (especially if they repeat more than once) — is unlikely to be due to chance.
  • These 2 clues almost always provide me with the answer as to whether there are at least some P waves that are being conducted. That said — today’s case was an exception for me, in that: i) The slight variation in R-R interval that we see in Figure-1 has no apparent “pattern” of group beating (and I see no specific R-R intervals that are clearly much shorter than all of the others); and, ii) Although a number of PR intervals look similar — they do not appear to be the same.

BOTTOM LINE: At this point in my interpretation — I did not know for certain what the mechanism of today's rhythm was. All I could say at this point was the following:
  • The rhythm was supraventricular — and almost (but not completely) regular.
  • The ventricular rate was ~70-80/minute.
  • There were lots of P waves — some of which were definitely not conducting — but others which might be conducting.
  • I knew that I’d need to use calipers to figure out more.

PEARL #2: When confronted with a rhythm in which some P waves may be conducted, but other P waves are definitely not conducted (as is the case in Figure-1) — I have found the simple step of labeling P waves to be incredibly helpful (Figure-2).
  • The reason why labeling P waves is so helpful — is that it instantly facilitates determining which P waves are (or are not) likely to be conducted.
  • Recognizing that 1 or more P waves are not conducted raises the possibility of some form of AV block.
  • KEY Point: For there to be some form of AV block — the atrial rhythm should be regular (or at least almost regular). Clearly there are exceptions to this general rule (ie, in addition to some form of 2nd-degree AV block — there may also be PACs, PVCs, echo beats, etc.) — but once you establish that there are dropped beats and an underlying regular (or at least fairly regular) atrial rhythm — the possibility of AV block becomes much greater.

  • On the other hand — recognizing that the atrial rhythm is not regular, especially when P wave morphology is not always uniform — instantly tells you that something other than typical AV block is occurring.

Figure-2: I've labeled the P waves from Figure-1. Using calipers makes this EASY. Since the first few RED arrows are obvious — I simply set my calipers to the P-P interval suggested by these first few RED arrows. This allowed me to "walk out" regular atrial activity throughout the entire rhythm strip (and allowed me to confirm that the "extra peaking" of the T waves of beats #4, 8, 10 and 12 — was clear indication of an "on-time" P wave hidden below).

My THOUGHTS at This Point:
  • The labeled P waves in Figure-2 confirm an underlying regular atrial rhythm at a rate slightly greater than 100/minute.
  • The rhythm in Figure-2 is supraventricular (narrow QRS). Caliper measurement of R-R intervals confirms slight-but-definite irregularity of the ventricular rhythm. I saw no clear pattern to this slight irregularity in the ventricular rhythm (ie, no "group" beating).

  • There are many more P waves than QRS complexes (ie, 21 RED arrows, compared to the 14 QRS complexes seen on this tracing). This confirms non-conduction of a number of on-time P waves — which suggests some form of AV block.

  • I thought complete AV block to be unlikely given irregularity of the ventricular rate (Most of the time — the ventricular rhythm will be regular when there is complete AV block).
  • The Mobitz II form of 2nd-degree AV block is uncommon. It seemed highly unlikely here — given the narrow QRS and absence of consecutive P waves with the same PR interval.

  • By the process of elimination — the rhythm in Figure-1 most likely represents some form of Mobitz I, 2nd-degree AV block ( = AV Wenckebach). But because of the atypical features of today's arrhythmia — I would need to construct a laddergram to prove my suspicion.
  • KEY Point: Even though I was unable at this point to come up with a definitive rhythm diagnosis — the above deductions provide sufficient information for appropriate initial clinical management: i) Some form of Mobitz I, 2nd-degree AV block is likely; and, ii) A pacemaker is not needed — since complete AV block is unlikely, and the overall ventricular rate is more than adequate.


Deriving the LADDERGRAM:

The complex mechanism of today's case is best explained by step-by-step derivation of a Laddergram (See ECG Blog #188 for review on how to read and/or draw Laddergrams).

  • NOTE: Today's case provides an example in which I needed to construct a valid laddergram explanation in order to "solve" the arrhythmia.
  • Sequential legends over the next 10 Figures illustrate my thought process as I derived this laddergram.

Figure-3: It is usually easiest to begin a laddergram by marking the path of sinus P waves through the Atrial Tier (RED lines drawn directly below the onset of each of the P waves — as shown by the large BLUE arrows). Note that these RED lines in the Atrial Tier are nearly vertical — since conduction of sinus P waves through the atria is rapid. Note also that the P-P interval between successive P waves (vertical RED lines) is equal!

Figure-4: The most challenging part of most laddergrams is construction of the AV Nodal Tier — so I generally save that for last. Therefore, after drawing in all P waves into the Atrial Tier — I prefer to next add indication of all narrow QRS complexes into the Ventricular Tier. The large BLUE arrows show that my landmark for QRS complexes in the laddergram is the onset of the QRS. Note that the RED lines in the Ventricular Tier are also nearly vertical — since conduction of these narrow QRS complexes through the ventricles is rapid.

Figure-5: It's time to begin "solving" what we can in the laddergram. I do this by connecting those P waves in the Atrial Tier that might logically be conducting to narrow QRS complexes in the Ventricular Tier (slanted BLUE lines within the AV Nodal Tier). Although the PR intervals preceding beats #1, 4, 6, 8, 10, 12 and 14 are not all quite equal — Doesn't it appear as if the angle of slant for these BLUE lines in the AV Nodal Tier looks similar? — and — Doesn't there seem to be a "pattern" for these BLUE arrow P waves that do appear to be conducting?

Figure-6: It's time to assess the mechanism we are proposing. IF my initial assumption (ie, that the P waves preceding beats #1,4,6,8,10,12 and 14 are all conducting) — HOW might you most logically connect the unattached atrial lines with the unattached ventricular lines in this Figure-6?
(NOTE: In the 1st grouping [that encompasses beats #1,2,3] — there are 3 unattached atrial lines and 2 unattached ventricular lines. In each of the 5 groupings that follow — there are 2 unattached atrial lines and just 1 unattached ventricular line).

Figure-7: I thought it easiest to address the 1st grouping in this rhythm (that encompasses beats #1,2,3). Doesn't it seem logical to postulate a 4:3 AV Wenckebach cycle for this 1st grouping — in which the PR interval for the RED, PURPLE and GREEN P waves progressively increases — until the YELLOW P wave is blocked. The cycle then begins again with the RED P wave that precedes beat #4, which conducts with a shorter PR interval.

Figure-8: I then addressed the next 5 groupings. The slanted PURPLE lines within the AV Nodal Tier show the only logical option for connecting the unattached atrial lines with the only unattached ventricular line in each grouping.

Figure-9: This means that the single remaining unattached atrial line that we saw for each grouping in Figure-8, must be blocked (slanted YELLOW lines with the butt end that do not make it out of the AV Nodal Tier).

Figure-10: For clarity — I've colored with PURPLE arrows those P waves in the lead II rhythm strip that correspond to the PURPLE lines passing through the Atrial, AV Nodal and Ventricular Tiers in the laddergram. 

Figure-11: For clarity — I've colored with YELLOW arrows those P waves in the lead II rhythm strip that correspond to the YELLOW lines that are blocked, and do not make it out of the AV Nodal Tier.

Figure-12: For my final laddergram illustration — I've colored all laddergram elements in RED. If you step back a little bit from this final illustration — it should be apparent that the mechanism of today's rhythm is 2nd-degree AV block, Mobitz Type I ( = AV Wenckebach) — with 4:3 AV conduction for the 1st grouping (that encompasses beats #1,2,3) — and with 3:2 AV conduction for the remaining 5 groupings.

Final COMMENT on Today's Case:

There are a number of reasons why the rhythm in today's case is so challenging. These include:

  • The atrial rate is rapid — with many of the P waves at least partially hidden either within the ST-T wave, or coinciding with the next QRS complex.
  • The ventricular rate is faster than is usually seen with AV block. This makes it difficult to tell which P waves are conducting to which QRS complexes.
  • The ventricular rhythm is irregular, and without an identifiable pattern of group beating.
  • Although a number of PR intervals look to be of similar duration — when measured with calipers, most of these PR intervals manifest slight-but-real differences in PR interval duration. This makes it difficult to tell which of these P waves may be conducting.

  • PEARL #3: "Not all patients read the textbook" before they develop their arrhythmia. Some cardiac rhythms simply do not follow the rules. It could be that this patient's acute pulmonary problem resulted in hypoxemia that led to the atrial tachycardia and the unexpected variation in PR interval duration. Enhanced autonomic tone (which is common in acutely ill patients) — is another potential reason why the 1st conducted beat in each grouping showed such a variety in PR interval duration.

  • Final POINT: As emphasized earlier — Even though I was unable to be certain of the etiology of today's rhythm until I drew my laddergram — Application of the Ps, Qs, 3R Approach allowed me within seconds to strongly suspect some form of Mobitz I, 2nd-degree AV block — and — to determine that a pacemaker was unlikely to be needed (since Mobitz II and complete AV block were essentially ruled out — and the overall ventricular rate was more than adequate to maintain perfusion).


Acknowledgment: My appreciation to Farhan Khan (from Karachi, Pakistan) for the case and this tracing.

Relevant LINKS to Today's Case:

Additional Relevant Material to Today's Case:
  • See ECG Blog #185 — for review of the Systematic Ps, Qs, 3R Approach to rhythm interpretation.

How to Draw a Laddergram (Step-by-Step Demonstration) 
  • See ECG Blog #69 — for a Step-by-Step description on drawing a Laddergram.
  • See ECG Blog #188 — for a brief ECG Video review on the basics of what a Laddergram is — with LINKS at the bottom of the page to more than 50 ECG blog posts in which I review illustrative laddergrams.
  • See ECG Blog #164 — for a user-friendly rhythm-solving approach to AV Wenckebach, followed by Step-by-Step construction of the Laddergram.
  • ECG Blog #236 — Reviews in our 15-minute Video Pearl #52 how to recognize the 2nd-Degree AV Blocks (including "high-grade" AV block)
  • ECG Blog #186 — Reviews when to suspect 2nd-Degree, Mobitz Type I.

  • CLICK HERE — to DOWNLOAD my Free PowerPoint Laddergram STENCIL for your use as desired.

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